As such a drill bit insert, a drill bit insert is known, in which a hard surface layer made of a sintered material of polycrystalline diamond which is harder than an insert body is coated on the end portion of the insert body made of a cemented carbide. Here, U.S. Pat. No. 5,575,342 proposes: a drill bit insert in which such a hard surface layer coated on an end portion of the insert body having a columnar rear end portion and the hemispherical end portion with an outer diameter decreasing toward a tip side; and a drill bit to which the drill bit insert is attached such that the rear end portion of the insert body is buried in a fitting hole formed in the end portion of the bit body. In addition, U.S. Pat. No. 3,141,746 discloses a method for producing such a polycrystalline diamond sintered material, and in U.S. Pat. Nos. 3,913,280 and 3,745,623 disclose a manufacturing apparatus.